This invention relates to a frequency response correction system, and, more particularly, to a system utilizing a combination of circuit stages configured to phase-interact with one another and compensate for the frequency response of a planar diaphragm speaker.
A variety of planar diaphragm loudspeakers have been developed in recent years using differing materials and having differing constructions and configurations. In general, such planar loudspeakers typically include a relatively stiff and substantially planar diaphragm that is coupled at its rear surface to a loudspeaker driver. The driver presses on the rear surface of the diaphragm and causes sufficient vibration of the diaphragm to efficiently produce sound. Generally, the frequency response of a planar loudspeaker is determined by the type and density of the material used for the diaphragm, and the area, thickness and contour of its sound producing region, as well as the type, position and configuration of the driver. Each of these parameters is chosen in an attempt to achieve an acceptable degree of fidelity in the reproduction of sound in both the low and high frequency ranges.
Some of the advantages provided by planar loudspeakers over other types of loudspeakers include greater dispersion of sound and economy of manufacture. A further advantage of certain planar loudspeakers is that the front surface of the diaphragm can be molded or finished to take on the appearance of a relatively large acoustic tile, permitting unobtrusive installation of the loudspeaker in ceilings of commercial structures formed of like-appearing acoustic tiles as part of a distributed sound system. Alternatively, the front surface of certain planar loudspeakers can be molded smooth and flat and installed in an architectural ceiling or wall in such a manner that the front surface of the planar diaphragm is flush with the front surface of the ceiling or wall. This type of installation of planar loudspeakers in walls or ceilings enables a common decorative finishing material to be applied to the diaphragm and surrounding ceiling or wall surface, thereby making the loudspeaker non-visible from the exterior side of the wall or ceiling. A number of such diaphragms can be joined together in a contiguous and seamless array to create a sound screen upon which video images can be projected as part of a home theater as shown and described in U.S. Pat. No. 5,007,707, which is assigned to the same assignee as the present application.
To comply with building and safety codes, the individual planar diaphragm loudspeakers of a distributed sound system may have to be surrounded on the rear side by a sealed metal enclosure or box. Whenever installed in an architectural wall or ceiling, whether or not in a separate sealed enclosure, there is usually a severe limitation in the depth of air space behind the planar diaphragm relative to the surface area of the diaphragm, which creates unusual and adverse acoustic conditions. These conditions typically result in an unacceptably high system resonant frequency (Fr), as well as an unacceptably high system resonant Q (Qf). As a consequence, a response peak typically occurs in a mid-bass region, and low bass frequency response is typically deficient. For example, the response peak for a planar diaphragm loudspeaker in an air chamber having a limited depth dimension might be in the range of 125 to 200 Hz., whereas preferably it would be in the range of 25 to 50 Hz.
The degree to which Fr and Qf parameters are non-optimal varies with specific planar diaphragm speaker design characteristics and the air chamber behind such speaker. In general, a product line might include several planar diaphragm speakers having different size diaphragms, and each of those speakers may have several different metal enclosures or boxes from which to choose depending on where the speaker assembly is installed. It would be desirable, therefore, if the signal compensation for non-optimal Fr and Qf parameters could be calibrated to the specific planar diaphragm speaker/air chamber combination.
Another characteristic of planar diaphragm speakers mounted in air chambers with a limited depth dimension is that they often exhibit an integrated power response decline in a mid-treble region (e.g., about 5 kHz.) and an integrated power response rise in a high-treble region (e.g., above 10 kHz.), which in turn degrades mid-range and treble reproduction accuracy. Again, the degree to which such mid-treble and high-treble responses are non-optimal varies with specific planar diaphragm speaker design characteristics and the associated air chamber. Signal compensation for non-optimal mid-treble and high-treble characteristics preferably should also be calibrated to the specific planar diaphragm speaker/air chamber combination.
One known way of compensating for the frequency response characteristics of loudspeakers involves use of graphic and parametric equalizers. However, such equalizers require intricate and painstaking alignments at multiple frequency points since the adjustment of one frequency band tends to interfere with other frequency band adjustments, making it difficult to set relatively sharp frequency cut-offs. Moreover, such equalizers are relatively expensive. Consequently, the use of such equalizers is not considered to be a very convenient or desirable solution to the problem of compensating for the above-described frequency response characteristics of planar diaphragm speakers mounted in air chambers with a limited depth dimension. This is particularly so for a distributed system of planar diaphragm speakers in which there might be a variety of different planar diaphragm speaker/air chamber combinations, each with its own compensation requirements.
Another way of compensating for the frequency response characteristics of planar diaphragm loudspeakers is described in co-pending application Ser. No. 09/099,049. This system incorporates cascaded equalization circuits and includes, among other elements, a multi-section switch in a resonant circuit to enable single-control selection of pre-set amplitude (A), frequency (F) and bandwidth (Q) parameters corresponding to various enclosure depths. As a practical matter, however, this system provides frequency compensation characteristics that are more suited to a home theater application than to distributed sound applications of planar diaphragm speakers.
Accordingly, there is a need for a method and apparatus for compensating for one or more of the above deficiencies in the frequency response of planar loudspeakers when mounted in air chambers with a limited depth dimension that can be calibrated for a specific planar diaphragm speaker/air chamber combination in a simple and cost effective manner. The present invention fulfills these and other needs.
Briefly, and in general terms, the present invention resides in a novel system for compensating the frequency response characteristics of a planar diaphragm speaker mounted in an air chamber with a limited depth dimension. The system may include one or more unconventional frequency compensation stages or circuits for processing an audio source signal applied to a planar diaphragm speaker/air chamber combination. The system also may be implemented in a manner that easily and economically allows calibration or adjustment of the frequency compensation characteristics of the system to accommodate a variety of different planar diaphragm speaker/air chamber combinations.
More specifically, the present invention provides electronic compensation, in an unconventional manner, for unacceptably high system resonance frequency and system resonant Q parameters of a planar diaphragm speaker mounted in an air chamber having a relatively small depth dimension. The present invention also may provide electronic compensation for a decline in integrated power response in a mid-treble region and a rise in integrated power response in a high-treble region of a planar diaphragm speaker.
In a presently preferred embodiment, and by way of example only, the compensation stages or circuits of the system of the present invention may be derived from a modified second-order, high-frequency high-pass filter stage and a linear frequency path in an additive manner, and a signal derived from a mid-frequency gyrator stage in a subtractive manner, so as to phase-interact with one another and provide a corrective transfer function. Such transfer function serves to correct the unacceptably high system resonant frequency (Fr) and system resonant Q (Qf) parameters that occur in planar diaphragm speakers mounted in air chambers having a relatively small depth dimension.
The above modified second-order, high-frequency high-pass filter may be eliminated, substituted by a non-modified high-pass filter, or substituted by other order modified or non-modified high-pass filters. In addition, an underdamped high-pass filter stage may be applied to the source input signal as a means to further enhance low bass performance in a frequency region below Fr. In an alternative embodiment, such underdamped filter stage may be applied to the system output signal. The transfer function of the compensation circuits also may serve to correct the integrated power response decline in a mid-treble region and the integrated power response rise in a high treble region typical of planar loudspeakers mounted in air chambers with a limited depth dimension. Each stage or circuit may be implemented in either the analog or digital domain.
In a further aspect of the present invention, the system may be configured to allow or provide for a plurality of frequency response compensation characteristics, each adapted or calibrated to optimize a specific planar diaphragm speaker/air chamber combination. This may be accomplished by substitution or adjustment of one or more components of the circuitry in order to tailor the system response for a specific planar diaphragm speaker/air chamber combination. In a preferred embodiment, for example, selected circuit components may reside on one or more auxiliary members in the form of parts carriers or xe2x80x9cdaughterxe2x80x9d boards or other structures that can be plugged into or otherwise releasably connected to a main or xe2x80x9cmotherxe2x80x9d board where the remainder of the frequency compensation circuitry resides. Each parts carrier or board may comprise circuit components with values that determine the response parameters of at least one of the above-described stages of the system of the present invention. Preferably, the parts carrier or daughter board will include passive circuit components only, and a single parts carrier or daughter board may include components for each of the stages or circuits that need to be calibrated or adjusted for a particular planar diaphragm speaker/air chamber combination. An appropriate number of such parts carriers or boards can be devised to accommodate all of the combinations of planar diaphragm speakers and metal enclosures or boxes (or other air chambers) in a product line. Thus, by plugging or otherwise connecting a parts carrier or daughter board to the main board, the system can be calibrated or adjusted to a specific planar diaphragm speaker/air chamber combination. Alternatively, a multi-section switch for selecting such circuit component values, or combinations of values, may substitute for the parts carriers or boards, if desired.
These and other advantages of the invention will become apparent from the following detailed description of the preferred embodiments, taken in conjunction with the accompanying drawings, which illustrate, by way of example, the principles of the invention.